314 THE CRUSTACEA, § 268. 



1. With the Myriapoda, these appendages are ambulatory and have the 

 same form with all the segments of the body ; and only with the Chilopoda 

 the anterior and middle pairs of the first segment corresponding to a 

 thorax, are changed into tactile organs. 



2. With the Isopoda, Laemodipoda, and Amphipoda, the first thoracic 

 pair are tactile organs. ^Vith the Amphipoda, the second and third 

 thoracic pairs are changed into prehensile organs armed with a claw. The 

 five anterior abdominal pairs are ambulatory and unchanged, with the Iso- 

 poda, and Amphipoda. But the remaining posterior pairs are transformed, 

 with the first of these orders, into lamelliform respiratory organs ; and iviii. 

 the second, into short, very movable appendages, terminated, each, by a 

 double uni- or multi-articulate cirrus, which serve sometimes as oars, some- 

 times as gyratory organs. 



3. With the Decapoda, the thorax is eftirely abortive, and its three 

 pairs are changed into oral and tactile organs ; while the first pair, belong- 

 ing to the anterior abdominal segments, is usually transformed into a for- 

 ficulate jjrehensile organ. The four succeeding pairs are simply ambulatory 

 organs. But the appendages of the posterior part of the abdomen are re- 

 duced to tendril-like processes, which play a part in the act of generation. 



With the Scjuillina, the three thoracic, and the first two abdominal pairs 

 have the form of prehensile organs, while the three succeeding pairs retain 

 their character of ambulatory organs, and those of the remaining posterior 

 segments are changed into lamelliform fins. 



4. In the section of the inferior Crustacea, designated usually under the 

 name of Entomostraca, the head and thorax are fused into a single part 

 called Cephalothorax, and the mouth is situated so far behind, that the first 

 pair of feet is in front of it. The locomotive apparatus here consists 

 usually of oars or prehensile organs. With the Poecilopoda, the first 

 three pairs of appendages are forficulate, as, also, are the three pairs of 

 jaws. With the Phyllopoda, and Lophyropoda, the first two pairs of feet 

 resemble antennae ; of these sometimes the first, as well also as the second, 

 which are usually branched, serve as oars ; '^' here, also, the often very 

 numerous, anterior abdominal appendages are used usually as fins, while the 

 posterior ones are scarcely at all developed. 



5. With the Cirripedia, the first thoracic pair is transformed in a 

 remarkable manner. \Vith the Lepadea, they are changed into a soft 

 foot; and with the Balanodea, into a shell. '^^ The remaining six pairs 

 are multiarticulate cirrate organs, and the abdomen is prolonged into a tail 

 free from appendages. The three anterior pairs of these cirrate organs are 

 the shorter, and have a tactile function ; while the three posterior are used 

 as gyratory organs. 



b. With the Siphonostoma, the mouth is even still further behind, and 



S With Cyclnpa, Cyclopsina, and Cypris, it is the younjj animals, these organs are used clearly as 



the first pair of feet that is transformed into oar- oars; see Jurine, Hist. d. Monocles, loc. cit. PI. 



like Organs, l)ut it is the second with Apus, Lim- XX. fifj- 9i and PI. XXI. fig. 1, 2{ChirocephaluK)^ 



nadia, Daplinin, ■Am\ Polyphemus. and Joly, in the Ann. d. So. Nat. XIII. PI. VII. 



With the Hranchiopoda, the disposition is quite {Artcmia).* 



different. Tlie anterior pair is clianged into two 4 See Thompson, Zool. Research. PI. IX. fiR. 3 



«lifhtly movable appendages, hooli-like or digiti- (Ba/aniis), and ßurmeister, Beitr., &c., Taf. I. 



form, and rolled spirally. With the embryos and lig. 3-5 (_Lepas). 



♦ [ 5 268, note 3.] The first pair of feet is, gene- cussion of this point, see Dana, Report on Crustac. 

 rally, the second pair of aulcunae. For a full dis- &c. p. 1001. — Ed. 



